Light olefins are important raw materials in many petrochemicals because they are building blocks for many end products, such as polyethylene and polypropylene. Recently, market analysis show that the demand for propylene is outpacing that of ethylene and the current supply cannot match the demand. A large proportion of propylene is produced primarily by steam cracking (SC) of light naphtha and secondarily by fluid catalytic cracking (FCC) process.
SC is an established technology for the production of light olefins, such as ethylene and propylene. It accounts for about 60-65% of the world's propylene production, with the established refinery FCC process accounting for about 30% and the remainder being produced on purpose using metathesis chemistry or propane dehydrogenation.
With the ethylene and gasoline being the main products from SC and conventional FCC, respectively, propylene and other light olefins may be obtained as byproducts from these technologies. Propylene may be produced by cracking heavy liquid hydrocarbons; while cracking ethane typically produces almost no propylene. Most modern steam crackers use ethane feedstock, as recently ethane feedstock became more abundant from shale gas, leading to less propylene being produced via SC plants.
On-purpose propylene production technologies, such as propane dehydrogenation and metathesis, may be used to bridge the propylene demand gap. However, the cost associated with these technologies remains less competitive relative to steam crackers and FCC. Additionally, new FCC catalysts involving the addition of ZSM-5 catalyst and new technologies such as DCC (Deep Catalytic Cracking), CPP (Catalytic Pyrolysis Process), high severity FCC cracking (e.g. Indmax®, PetroFCC®) may also be used in the FCC process to produce more olefins at the expense of gasoline production. Table 1 illustrates various olefin production methods and amount of gasoline produced in each process.
TABLE 1Olefin production methods.FCCDCCCPPNaphtha-SCReaction temp/° C.500-550530-590560-670760-870Reactor press/barg1-31, 211-0Residence time/s1-5 1-101-30.1-0.2Catalyst/oil ratio (wt/wt)4-810-1515-25—Dispersion steam (%)1-3 5-3030-5030-80Cracking environmentRiserRiser and bedRiserCoilReaction mechanismCarboniumCarboniumFree radicalFree radicaland carboniumPropylene yield wt %3-410-2015-2512-14Gasoline yield wt %50-5522-2812-1512-18
In comparison with FCC; the DCC and CPP reactor operating temperatures are higher, therefore, DCC and CPP require higher regeneration temperature to provide the heat of reaction. Catalyst to oil ratio are also 2 to 3 times higher. DCC and CPP use more steam than conventional FCC (Table 1) and their operation is sometimes termed as steam catalytic cracking (SCC). SCC is a process of cracking hydrocarbons to light olefins in mild temperatures in the presence of steam over a catalyst. SCC combines mild thermal cracking with the acid promoted cracking of a zeolite-based catalyst, and can provide very high yields of light olefins (with the possibility of varying the propylene-to-ethylene ratio) while operating at temperatures much lower than those used in the SC process
FCC/DCC/CPP reactors use <100 micron size zeolite catalyst in a fluidized bed circulating at essentially atmospheric pressure and high temperatures (e.g. >550° C.) with added dilution steam to lower the partial pressure of the HC and reduce coke formation. In these reactors, the major conversion reactions are:Paraffins→Smaller paraffins+OlefinsAlkyl Naphthene→Naphthene+OlefinAlkyl Aromatic→Aromatic+OlefinMulti-ring Naphthene→Alkylated naphthene with fewer rings
Also, at the reactor conditions, the following reactions occur:CH4↔C+2H2 HC decompositionand in the presence of steam the following reactions occur, to small extent depending on the effectiveness of the metals content of the catalyst and that in the HC feedstock:C+H2O↔CO+H2 CO+H2O↔CO2+H2 Water Shift ReactionCH4+H2O↔CO2+H2 Steam ReformingC+CO2↔2Co Boudouard Reaction
The above reactions account of the formation of CO and H2 on the FCC/DCC/CPP catalyst. It is known that once CO and H2 are present, they will react in the presence of catalyst to form hydrocarbons, and oxygenates; this is the principle of the Fischer-Tropsch (FT) process. Also, that, during high temperature combustion, oxygenates are formed too besides CO, CO2 and H2O.
It has been reported that oxygenates are found in FCC reactor process stream outlet in concentrations ranging from 10-2000 ppm, but no elaborate investigations were reported; this is because FCC plants were originally designed primarily to produce gasoline; and oxygenates are beneficial addition to the gasoline and were not considered as contaminants. In fact, many FCC plant operators are not aware of the formation of oxygenates because these components are not measured or tracked at their production facilities.
DCC/CPP reactors on the other hand operate with higher steam dispersion to hydrocarbon ratios that give rise to high CO concentration, and produce significantly more olefins (ethylene, propylene and butylenes), which leads to higher concentration of oxygenates in the separated olefins streams representing contaminants that have to be removed to concentrations in the low single digits ppm level upstream of product towers, hydrogenation reactor, or alkylation unit.
Mechanism of oxygenates formation in Fischer-Tropsch process at high temperature (e.g., ˜300° C.) and pressure (e.g., ˜40 bar) in the presence of oxides of Fe, Co, Cu, Cr, etc. on alumina catalyst; and in combustion of fuels at 1 bar and >1000° C. are well researched and reported and give a guide to how oxygenates are formed in FCC/DCC/CPP reactors.
Oxygenates are formed in FCC reactors where heavy petroleum hydrocarbon with added steam are catalytically cracked over zeolite catalyst, such as ZSM-5 catalyst at temperature of 550-650° C. and atmospheric pressure, to produce a mixture of lighter paraffinic, olefinic and aromatic compounds. Side reactions of CO and H2 in contact with zeolite catalysts produce a mixture of oxygenates include organic acids, alcohols and carbonyls in concentrations ranging from 10-1000 ppm, for example, depending on the type of feedstock, catalyst, ratio of dispersion steam to hydrocarbons, and cracking temperature.
The mechanisms of oxygenate formation involve complicated reactions, simplified net catalytic reactions that result in the formation of oxygenates are:C+H2O→CO+H2 Coke Conversion to COCO+H2O→HCOOH Formic AcidHCOOH+H2→HCHO+H2O FormaldehydeC2H4+H2O+CO→CH3CH2COOH Propionic AcidCH3CH2COOH+H2→CH3COCH3 AcetoneCH4+CO+H2O→CH3COOH+H2 Acetic AcidCH3COOH+H2→CH3CHO+H2O AcetaldehydeCO+2H2→CH3OH MethanolCO2+3H2→CH3OH+H2O MethanolC2H4+H2O→CH3CHO+H2 AcetaldehydeC2H2+H2O→CH3CHO Acetaldehyde
As shown in typical FCC block flow diagrams FIG. 1 and FIG. 2, the more water soluble oxygenates such as acids, and light alcohols as well as NH3, HCN are dissolved in the aqueous condensate upstream of the wet gas compressor (WGC); within the WGC condensate is separated at higher pressure with more alcohols and carbonyls being separated from the main hydrocarbon process stream; the remainder oxygenates and carbonyls compounds follow the C3's (e.g., propane and propylene) and C4's (e.g., butane, isobutene, and/or butylenes) streams in the fractionation train. Oxygenates and carbonyls in the C3's and C4's streams require removal if these streams are further processed catalytically to avoid deactivation of the used catalysts or are required to meet products specifications.
When removing acid gases with amine solution, aldehydes may be trapped. The aldehydes dissolved in the alkaline amine solutions react producing polyaldols by Aldol Condensation Reaction(s). These polymers known in the industry as “red oil” induce fouling of the amine absorber. Aldol Condensation Reactions result the liquid red oil formation, which is a reaction product of few numbers of aldehyde monomer, and further polymerization leads to the formation of high molecular weight red/yellow solid polymer. In the amine system, the acetaldehyde polymer will settle on internal equipment surfaces leading to fouling and eventual plugging. Fouling and plugging of the internal equipment means the unit must be shut down to perform cleaning. Every time a unit operation has to be shut down for cleaning it means that a cost is incurred due to lost production, over and above, the actual cost to clean the equipment.
The red oil Aldol polymer formed in the absorber will be carried to the amine regenerator which operates at much higher temperature (e.g., 110-115° C.), this causes accelerated further polymerization of the dissolved carbonyl compounds in the rich amine solution forming the solid aldol polymer that eventually result in it deposition and fouling of the regenerator reboiler.
The reactor effluents are also contaminated with sulfur compounds, mainly H2S and mercaptans (RSH), formed in the FCC reactor. H2S is removed from liquefied petroleum gas (LPG) by contacting it with amine solution; and the H2S-depleted stream is then contacted with an aqueous caustic solution in a Mercaptan Removal Unit.
A description of an example mercaptan removal unit follows. In the mercaptan removal unit, the LPG enters the mercaptan extractor, may operate between 30-40° C., for example, where it intimately contacts the caustic solution to extract the mercaptan (RSH) from the LPG and form mercaptide (RSNa). The mercaptan extracted LPG may exit the extractor. The caustic solution may leave the bottom of the mercaptan extractor (“rich” caustic) and may then be injected with proprietary liquid cobalt phthalocyanin catalyst, heated to an elevated temperature (e.g., 55-60° C.) and injected with compressed air before entering the oxidizer vessel where the RSNa are converted to disulfides oil (DSO). The oxidizer vessel has a packed bed to keep the aqueous caustic and the water-insoluble disulfide well contacted and well mixed. The caustic-DSO mixture then flows into the separator vessel where it is allowed to form a lower layer of “lean” caustic and an upper layer of DSO. The disulfides are withdrawn from the separator and routed to fuel storage or to a hydrotreater unit. The regenerated lean caustic is then pumped back to the top of the extractor for reuse.
Carbonyls in the LPG which enter the caustic extractor are transferred from the organic hydrocarbon phase into the aqueous caustic phase and react with the caustic solutions producing polyaldols polymers by Aldol Condensation Reaction(s). This results in formation of a water insoluble polymer known in the industry as “red oils” and induces fouling by coating the surfaces of the caustic extractor, and downstream caustic handling equipment which reduce the operation efficiency of the caustic systems. Aldol condensation reactions result the liquid red oil formation, which is a polymerization product of aldehyde monomer. Further polymerization may lead to the formation of high molecular weight red/yellow solid polymer. The aldehydes are more reactive than ketones; thus the remaining carbonyls, mainly ketones, are carried through in the mercaptan-depleted C3/C4 LPG stream.
The rich caustic solution leaving the extractor loaded with mercaptide, aldol polymer and dissolved hydrocarbon component each to the limit of its solubility in the aqueous phase. The feed LPG may also contain the highly unsaturated butadiene which has large solubility in the aqueous phase. This rich caustic solution now with added cobalt ions, saturated with oxygen and heated to elevated temperature (e.g., 55-60° C.) provides enhanced conditions for Aldol polymerization of the dissolved carbonyls and the addition polymerization of the dissolved butadiene monomer.
FCC reactors operated in the gasoline mode may form low levels of oxygenates and diener contaminants while FCC reactors operated in the olefins mode may have increased concentration of CO and H2O. Elevated levels of CO and H2O in the reactor may increase the concentration of oxygenates and dienes in the reactor effluent by many orders of magnitude compared to a reactor with lower levels of CO and H2O. For LPG generated in the FCC operated in the gasoline mode, the carbonyls and dienes may be effectively removed in the H2S removal amine or weak alkaline extractor upstream of the mercaptan removal unit. In contrast, for LPG generated in the FCC operated in the olefins mode, the carbonyls and dienes are largely passed to the mercaptan removal unit which may cause fouling of the extractor and may lead to severe fouling of the oxidizer.